<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box{
            width:300px;
            height:8px;
            border:1px solid red;
            border-radius: 4px;
        }
        .inner{
            height:100%;
            width:20%;
            background-color: red;
            border-radius: 4px 0 0 4px;

            transition:all 5s;
        }
        .box:hover .inner{

            width:100%;
            border-radius: 4px;
        }
    </style>
</head>
<body>
<div class="box">
    <div class="inner">

    </div>
</div>

</body>
</html>